The invention relates to semiconductor amplifier circuits and, more particularly, to a differential amplifier circuit with an unlocking device.
Differential amplifiers are widely used in the telecommunications field. They allow for processing of weak signals conveying voice signals, and, more generally, data. A differential structure is particularly preferred in data transfer networks of wired networks (Wide Area Network) found in Asynchronous Transfer Mode-type (ATM) networks or Asynchronous Digital Subscriber Line-type (ADSL) networks as well as their principal derivatives HDSL (commonly designated by the generic term XDSL).
Generally, a differential structure has the effective advantage of eliminating harmonics and second-order non-linearities in distortion noise. Furthermore, a differential structure ensures greater immunity to common mode interference, such as the interference that power supply circuits of electronic circuits experience. Differential amplifier circuits, particularly those using complementary-type MOS type transistors, which are used in bi-CMOS technology, frequently use power supply sources. Such power supply sources often experience difficult start-up transients and blocking phenomena preventing the amplifier from operating.
Circuits for overcoming this problem and avoiding power supply source blocking in a semiconductor circuit are known. French patent application No. 2,767,976 entitled xe2x80x9cDispositif d""aide au dxc3xa9marrage pour une pluralitxc3xa9 de sources de courantxe2x80x9d discloses such a start-assisting device. The start-assisting device which, though not specifically adapted to an amplifier structure, is useful to supply devices for microprocessors and electronic apparatus. This device comprises a start-assisting device, which provides power supply sources with a start current for a transient period, until a steady state can be established. A complementary inhibiting device is thereafter required for steady-state operation.
Although differential amplifiers are especially important in integrated circuits used in telecommunications, it is still desirable to design start-assisting devices that are particularly adapted to their structure and can be easily and cost-effectively manufactured without requiring the addition of new circuits. However, a differential amplifier structure that directly and easily incorporates a suitable unlocking circuit is not yet available.
The invention is aimed at providing a differential amplifier structure having an efficient, simple unlocking device, which only requires the addition of a very limited number of other components.
Another object of the invention is to provide an unlocking device which is adapted to the architecture of a differential amplifier to be integrated into an integrated circuit, and, in particular, that uses CMOS-type components of bi-CMOS technology.
Another object of the invention provides a differential amplifier structure, which is adapted to telecommunication network requirements and, particularly, ADSL or HDSL-type links.
The invention achieves these objects through a differential amplifier structure having a first stage including first and second transistors of identical polarity, such as NMOS-type transistors, which are assembled to provide a differential amplifier. The first and second transistors are fed by first and second mirror current sources respectively, which are controlled by a control circuit supporting common mode. The common mode control circuit has two inputs receiving a reference voltage VCM and a voltage representative of the common mode voltage of the amplifier.
A second Miller stage comprises third and fourth transistors of an opposite-type from the former transistor, for example PMOS. The inputs of the third and fourth transistors receive output signals from the first stage, which are used to increase the open loop gain of the amplifier circuit and to set the gain-bandwidth product of the amplifier. An additional unlocking circuit is inserted between the common mode voltage and the Miller stage inputs. The additional unlocking circuit causes controlled conduction of the third and fourth PMOS-type transistors until the common mode voltage reaches a value which is significantly close to the reference value.
Thus, it is ensured that the Miller stage is set for conducting and, hence, the first and second current sources are set for conducting, which ensures the amplifier operation. Once the common mode voltage rises back up to a normal operational value, the unlocking circuit locks and re-establishes impedance between the Miller stage output and the common mode voltage, thereby enabling the amplifier to operate in a linear mode.
Preferably, the unlocking circuit is provided by two MOS-type transistors causing short-circuiting between the common mode voltage and the Miller stage inputs. The drain terminal of each NMOS transistor (which can also be NPN transistors) or unlocking transistor is connected to a corresponding gate of one of the third and fourth transistors. Furthermore, both unlocking transistors have a source connected to the terminal representative of the common mode voltage. The gates of the unlocking transistors are connected to a reference voltage VCM and cause the Miller stage transistors to conduct when the true value of the common mode is significantly smaller than the reference value VCM.
In a particular embodiment, the amplifier may comprise a cascade stage comprising, for instance, bipolar transistors. More particularly, the differential amplifier may comprise a first stage comprising a first and a second NMOS-type transistor (which can also be NPN-type transistors) assembled as a differential pair. The gates of the first and second NMOS-type transistors may receive input signals via feedback resistors from a common source connected to a third current source.
The differential amplifier may further comprise a second Miller gain stage comprising third and fourth PMOS-type transistors, for example. The third and a fourth PMOS-type transistors may be assembled as a common source, each associated with a current source and a capacitor. The second Miller gain stage output may be connected to output terminals and having one input.
The differential amplifier may further comprise fifth and sixth PMOS-type transistors, for instance. The fifth and sixth PMOS-type transistors provide the first and second current sources, respectively, feeding the first and the second NMOS-type transistors, which form the differential pair of transistors. The fifth and sixth transistors may be assembled as a common source and each having a drain. The drains of the fifth and sixth transistors are connected to corresponding drains of the first and second transistors in the differential pair, as well as a gate controlled by the common mode supporting circuit.
The differential amplifier may further comprise an unlocking circuit comprising seventh and eighth NMOS-type transistors, for example, having gates that receive a reference value VCM. The unlocking circuit may short-circuit the common mode voltage and the gates of the third and fourth transistors, which form the Miller stage when the common mode voltage goes below the reference value VCM.
The invention is especially adapted to designing wide-band amplifiers used in wired telecommunications networks, and, more particularly, to wired telecommunication networks found in the Asynchronous Digital Line Subscriber-type networks and their derivatives.